A NAND flash memory (referred to as “NAND memory” below) stores information according to a charge amount accumulated in a floating gate of a memory cell. A voltage (read voltage) is applied to the memory cell, and the stored information can be read according to the result.
The distribution of the charge amount accumulated in the floating gate of the memory cell may change from the initial state. Therefore, an optimal reading voltage (read voltage with less reading errors) may also change.